Hardened tools are typically used to work in various modes such as drilling, routing or general machining of a variety of softer materials. For example, tungsten carbide drills are used to drill through the glassy G-10 material of printed circuit boards, and nitrided steel router bits are used for cutting 4140 steel. Although machine shop lubricants are normally employed to assist in the operation of these tools, these lubricants merely serve as coolants since they are readily displaced at the actual cutting interface both physically and thermally. Thus, the outer atomic surface of the cutting tool is in intimate contact with the corresponding atomic surface of the material being cut at the maximum point of application of both stress and temperature. In such operation, without the benefit of any lubricant protection, the maximum opportunity for destruction of the tool occurs.
Lubricity is essential for reasonable life of materials subjected to such severe operational environments. At the elevated temperatures that occur in the tool's outer atomic layers during machining, conditions exist to promote physical as well as chemical deterioration of the tool's working surface. Lubricants in general function by allowing slippage between moving parts by providing shearing action and floating support as long as the lubricant is not thermally destroyed or physically displaced. Even after the lubricant film is penetrated, there is still a measure of protection remaining in the form of metal oxides that may occur on the tool's surface. Once the film is penetrated, however, there is nothing to prevent various failure mechanisms such as cold welding, where the lattice structures of the tool and material being cut actually join by atomic diffusion; burning; or chemical attack from activated reagents present in the material being cut. It is well known that for example, wood, although softer, is damaging to tungsten carbide routers and consumes these routers at an excessive rate. Similarly, diamond cutting tools fail rapidly when machining ferritic alloys by the dissolution of the carbon into the iron being cut following the natural tendency of iron and carbon to form the solid solutions that make the manufacture of steel possible. Likewise, the nickel and cobalt binders use in the manufacture of metal carbide cutting tools are susceptible to similar failure mechanisms. Nickel is extremely soluble in a variety of materials, particularly at the elevated temperatures at the cutting edge. Clearly, there is more to the protection of good cutting edges than hardness alone.
In spite of the foregoing factors, the emphasis on cutting tool development has been almost solely directed to hardness factors alone. Over the years the hardness inherent in metal carbide tools, despite an almost total lack of ductility, resulting in brittleness, has made carbide tools more popular than the softer, albeit more ductile, tools made of heat treated, hardened steel alloys. It has been desireable to improve even the metal carbide tools by using treatments that offer a more continuous, binderless surface of an even harder refractory material such as, for example, titanium nitride. Titanium-nitride films are deposited by chemical vapor deposition, sputtering or reactive ion plating. However, the results of these hard coatings are deficient for various uses.
One of these uses involves the problem of drilling holes in printed circuit boards. This problem is particularly aggravated by the increased demand for multilayered printed circuit boards which are constructed of alternating layers of G-10 (glass webbed material) and metal conductors. During the act of drilling, the glassy material actually flows under melt as the drill bit penetrates the board to the extent that "smearing" of the layers together occurs at the edges of the resulting hole. The need for hole desmearing processes has added extra steps into the process of printed circuit board manufacture and is a problem that has not been alleviated by titanium-nitride films and other refractory material coatings applied to tungsten carbide drill bits. Problems also exist for carbide router bits used in drilling processes. This problem is not that the carbide is softer than the glass that it is drilling since the carbide is in fact much harder, but a combination of surface affinity of the glass for the tool, heat generation and failure of the carbide tool itself. Therefore, a need exists for a lubricant for protecting two surfaces from themselves during high energy sliding contact.
It should be noted that the characteristics that make a lubricant effective include a low ability to wet, and subsequently the ability to bond to the surfaces. Good lubrication implies smooth slippage of one surface over another and will breakdown if the lubricant is displaced. A need thus exists for a material-working tool that possesses such lubricity as well as a method for bonding such lubricant to surfaces of the tool that will assure that the lubricant is not displaced by adhesion failure when the stresses inherent in the tool's operation are experienced.